High temperature capacitive mems pressure sensor

ABSTRACT

A MEMS pressure sensor includes a first plate with a hole on a diaphragm bonded to the first plate around its rim with the diaphragm positioned over the hole. An isolation frame is bonded to the diaphragm and a second plate with a pillar is bonded to the isolation frame around its rim to form a cavity such that the end of the pillar in the cavity is proximate a surface of the diaphragm. The diaphragm and second plate form a capacitive sensor which changes output upon deflection of the diaphragm relative to the second plate.

BACKGROUND

The present disclosure relates to a compact MEMS capacitive pressuresensor used primarily for sensing air pressures and air vehicleoperations, which is constructed to provide long term stability, reducedtemperature induced errors, and to provide a rugged and accurate sensingelement for a pressure sensor assembly.

Solid state capacitive type pressure sensors have been well known in theart, and are widely accepted because of their ability to beminiaturized, and to be made using batch fabricating techniques to holdcosts down. Such prior art sensors have used glass or semiconductorbases, and formed diaphragm layers joined together around the rim of thediaphragm with anodic bonding, glass frit bonding, metal diffusion andsimilar bonding techniques.

It has also been known in the art to metalize borosilicate glass (soldunder the trademark Pyrex) layers for forming capacitive electrodes foruse with deflecting semiconductor diaphragms. Temperature stability is aproblem for pressure sensors used in air vehicles because they aresubjected to wide, quite sudden swings in temperature. Temperatureinduced stresses caused by materials which have different temperaturecoefficients continues to be a problem. Sensing elements that canwithstand temperature environments exceeding those in state of the artwould be useful.

SUMMARY

A MEMS pressure sensor includes a backing plate with a central hole on acircular diaphragm bonded to the backing plate around its rim with thediaphragm positioned over the hole. An isolation frame is bonded to thetop of the diaphragm and an electrode with a central pillar is bonded tothe isolation frame around its rim to form a cavity such that the end ofthe pillar in the cavity is proximate the upper surface of thediaphragm. The diaphragm and electrode plate form a capacitive sensorwhich changes output upon deflection of the diaphragm relative to theelectrode plate.

A method of forming a MEMS pressure sensor includes forming a firstbacking plate with a central hole and forming a diaphragm by creating adepression in the bottom side of a plate to create the diaphragm andbonding the bottom side of the diaphragm to the backing plate around itsrim. The method further includes forming an isolation frame and bondingthe isolation frame to the top side of the diaphragm around its rim. Themethod further includes forming an electrode plate with a central pillarin the bottom side of the plate and bonding the bottom side of thecircular electrode plate to the top side of the isolation frame suchthat the end of the central pillar projects into the cavity that formsand is proximate the top side of the diaphragm to form a closed cavity.Forming a metal electrode on the outside of the diaphragm and a metalelectrode on the top side of the electrode plate forms a capacitivesensor which changes output upon deflection of the top side of thediaphragm relative to the pillar on the electrode plate due to pressuredifferences communicated to the diaphragm through the hole in thebacking plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of an embodiment of a MEMS pressure sensor.

FIG. 2 is a cross-section of another embodiment of a MEMS pressuresensor.

FIG. 3 is a cross-section of a further embodiment of a MEMS pressuresensor.

FIG. 4 is a diagram of the steps of forming a MEMS pressure sensor.

FIGS. 5A-5K are cross-sectional views that illustrate production stepsof a MEMS pressure sensor.

FIG. 6 is a cross-section of another embodiment of a MEMS pressuresensor.

DETAILED DESCRIPTION

FIG. 1 is a cross-section of an embodiment of a high temperature MEMSpressure sensor. Pressure sensor 10 includes top electrode plate 11formed from an SOI structure comprising top silicon layer 12, oxidelayer 13, and bottom silicon layer 14. Metal electrode 15 provides anelectrical connection through via V to a bottom silicon layer 14. Topelectrode plate 11 is bonded to quartz frame spacer 16, such as by afusion bond for example, at interface 24. Top electrode plate 11 andquartz ring spacer 16 are bonded to silicon diaphragm 18 in aconfiguration such that a pedestal formed by bottom silicon layer 14 ontop electrode plate 11 is proximate a top surface of silicon diaphragm18. Silicon diaphragm 18 is bonded to silicon backing plate 22 to formMEMS pressure sensor 10. Metal electrode 15 on top electrode plate 11and metal electrode 20 on silicon diaphragm 18 allow MEMS pressuresensor 10 to function as a capacitive pressure sensor which changesoutput upon deflection of center web W due to pressure differencescommunicated through hole 23 in silicon backing plate 22 and chamber 25in silicon diaphragm 18. Silicon backing plate 22 is bonded to silicondiaphragm 18, such as by fusion bond 28 for example. Silicon diaphragm18 is bonded to quartz frame spacer 16 by fusion bond 26. Quartz framespacer 16 is also bonded to top electrode plate 11 by fusion bond 24.Chamber 17 may be evacuated and maintained under a vacuum. The hightemperature capability of MEMS pressure sensor 10 may be achieved due tothe thermal stability of the fusion bonds used to form MEMS pressuresensor 10. Metal alloys for electrodes 15 and 20 may comprise Al, Ti,Cr, TiW, and/or W alloys.

In the configuration of MEMS sensor 10 illustrated in FIG. 1, themaximum operating temperature may be about 800° C. (1472° F.).

FIG. 2 is a cross section of another embodiment of a high temperatureMEMS pressure sensor. MEMS pressure sensor 30 is similar to MEMSpressure sensor 10, except that in MEMS pressure sensor 30, electrodeplate 32 is formed completely from silicon instead of SOI material.Silicon electrode plate 32 with metal electrode 34 is bonded to quartzspacer frame 36, such as by fusion bond 44. Quartz spacer frame 36 isbonded to silicon diaphragm 38, such as by fusion bond 46. Siliconbacking plate 42 is bonded to silicon diaphragm 38, such as by fusionbond 48. Metal electrode 34 on silicon electrode plate 32 and metalelectrode 40 on silicon diaphragm 38 allow MEMS pressure sensor tofunction as a capacitive pressure sensor, which may change output upondeflection of center web W due to pressure differences communicatedthrough hole 43 in silicon backing plate 42 and chamber 45 in silicondiaphragm 38. Chamber 37 may be evacuated and maintained under a vacuum.The high temperature capability of MEMS pressure sensor 30 may beachieved due to the thermal stability of the fusion bonds used to formMEMS pressure sensor 30. Metal alloys for electrodes 34 and 40 maycomprise Al, Ti, Cr, TiW, and/or W alloys. As in MEMS pressure sensor10, the maximum operating temperature of MEMS pressure sensor 30 may beabout 800° C. (1472° F.).

FIG. 3 is a cross section of a further embodiment of a high temperatureMEMS pressure sensor. In this embodiment, top electrode plate 51 of MEMSpressure sensor 50 may be formed from SOI material as in MEMS pressuresensor 10 in FIG. 1. Top electrode plate 51 includes top silicon layer52 and bottom silicon layer 54 separated by oxide layer 55. Topelectrode plate 51 also includes conductive silicon layer 56 on thebottom of silicon layer 52 and sides of silicon pedestal 54. Metalelectrode 55 on top of silicon layer 52 is electrically connected tobottom silicon pedestal 54 as a result of silicon layer 56. Top siliconelectrode plate 51 is bonded to quartz spacer frame 57, such as byfusion bond 64. Quartz spacer frame 57 is bonded to silicon diaphragm58, such as by fusion bond 66. Silicon backing plate 62 may be bonded tosilicon diaphragm 58, such as by fusion bond 68. Metal electrode 55 ontop electrode plate 51 and metal electrode 60 on diaphragm 58 allow MEMSpressure sensor to function as a capacitive pressure sensor, which maychange output upon deflection of center web W due to pressuredifferences communicated through hole 63 in silicon backing plate 62 andchamber 65 in silicon diaphragm 58. Chamber 59 may be evacuated andmaintained under vacuum. The high temperature capability of MEMSpressure sensor 50 may be achieved due to the thermal stability of thefusion bonds used to form MEMS pressure sensor 50. Metal alloys forelectrodes 55 and 60 may comprise Al, Ti, Cr, TiW, and/or W alloys. Asin MEMS pressure sensor 10, the maximum operating temperature of MEMSpressure sensor 50 may be about 800° C. (1472° F.).

FIG. 4 is a flow diagram, and FIGS. 5A-5K are cross-sections,illustrating a method to form a MEMS pressure sensor. For ease ofexplanation, the method will be described with respect to forming MEMSpressure sensor 10 shown in FIG. 1. Procedure 90 is initiated byproviding an SOI wafer as shown in FIG. 5A (step 92). SOI wafer 5comprises top silicon layer 12 and bottom silicon layer 14 separated byinsulating oxide layer 13. In the next step of the process, the top andbottom layers of SOI wafer 5 are oxidized to form insulating layer 13 onSOI wafer 5 as shown in FIG. 5B (step 94). A pillar is then formed bymasking the bottom layer of wafer 5 and etching portions of the layer toform a pillar in bottom silicon layer 14 as shown in FIG. 5C (step 96).Etching may preferably be carried out by a dry reactive ion etch (DRIE).As shown in FIG. 5C, the etch is stopped by the middle oxide layer 13 inSOI wafer 5. The top and bottom oxide layers are then removed by etchingas shown in FIG. 5D (step 98). Etching in this step may, for example, becarried out by wet etching in HF solution.

In the next step, the top and bottom surfaces of SOI wafer 5 are dopedwith boron or phosphor, depending by Si wafer type (Boron for P type andPhosphor for N type), as shown in FIG. 5E (step 100) for better surfaceconductivity and reducing the effective capacitor gap. Boron/phosphormay be implanted by ion implantation or by diffusion. This stepcompletes the formation of top electrode plate 11 shown in FIG. 1.

Quartz frame spacer 16 is then formed to act as a standoff in the MEMSsensor structure of the invention shown in FIG. 1. Quartz frame spacer16 may be formed by ultrasonic machining and is shown in FIG. 5F (step102). Quartz frame spacer 16 may then be bonded to implanted, etched,planarized SOI wafer 5 as shown in FIG. 5G (step 104). Bond interface 24may be formed by a fusion bond.

In the next step, silicon diaphragm 18 is then formed from a siliconwafer by etching depressions in the top and bottom of wafer 18 as shownin FIG. 5H. Thinning wafer 18 results in the formation of flexible webW. Step S shown on the side of wafer 18 forms a base for futuredeposition of an electrical contact (step 106).

In the next step, the planarized bottom of quartz frame spacer 16 bondedto SOI wafer 5 is bonded to the planarized top of diaphragm 18 as shownin FIG. 5I (step 108). Bond 26 may be a fusion bond. The bondedstructure containing top electrode plate 11 (see FIG. 1) and diaphragm18 may then be bonded to silicon backing plate 22 with central hole 23as shown in FIG. 5J (step 110). Bond 28 may also be a fusion bond.

In the next step, top silicon layer 12 is masked and via 29 is formed byDRIE etching through top silicon layer 12 and oxide layer 13 as shown inFIG. 5K (step 112) Also shown in FIG. 5K, a portion of top electrodeplate 11 is removed by dicing the section of top electrode plate 11 overside step S on diaphragm 18 to allow deposition of an electrode on sidestep S (step 114). Finally, electrodes are deposited in via 29 on topelectrode plate 11 and on side step S on diaphragm 18 to create thestructure of sensor 10 as shown in FIG. 1 (step 116).

In an embodiment of the present invention, the quartz spacer under thetop SOI electrode plate in the MEMS pressure sensor of the invention isreplaced by a glass spacer. This structure is illustrated in FIG. 6.MEMS pressure sensor 70 includes SOI electrode plate 71 comprising topsilicon layer 72 and bottom silicon layer 74 separated by insulatingoxide layer 73. Top silicon layer 72 is bonded to glass spacer frame 76at bond interface 84 which, in turn, is bonded to silicon diaphragm 78at bond interface 82. Glass spacer frame 76 is bonded to top siliconlayer 72 and silicon diaphragm 78 by anodic or glass frit bonds known inthe art. Silicon diaphragm 78 is bonded to silicon backing plate 82 by(inventor, is this a fusion bond?). Metal electrode 74 on electrodeplate 71 and metal electrode 80 on silicon diaphragm 78 allow MEMSpressure sensor 70 to function as a capacitive pressure sensor, whichmay change output upon deflection of center web W due to pressuredifferences communicated through hole 83 in backing plate 82 and chamber85 in silicon diaphragm 78. Chamber 77 may be evacuated and maintainedunder vacuum. The maximum operating temperature of MEMS pressure sensor70 is less than the maximum operating temperature of MEMS sensors 10,30, and 50 of the present disclosure as a result of the glass spacer inthe structure, i.e. around 400° C. (752° F.).

Major benefits of the present invention include the high temperaturestability of the sensor resulting from the exclusive use of quartz andsilicon throughout the structure as well as the exclusive use of hightemperature fusion bonds. A further benefit results from the vacuumsealed chamber which eliminates costly prior art vacuum packagingprocess.

Discussion of Possible Embodiments

The following are non-exclusive descriptions of possible embodiments inthe present invention.

A MEMS pressure sensor may include a first plate with a central hole, adiaphragm bonded to the first plate around the rim of the first platewith the diaphragm positioned over the hole and an isolation framebonded to a top of the diaphragm. The pressure sensor may furtherinclude the second plate with a central pillar bonded to isolation framearound a rim of the isolation frame to form a cavity such that an end ofthe pillar in the cavity is proximate the first surface of the diaphragmand wherein the diaphragm and second plate form a capacitive sensor thatchanges output upon deflection of the diaphragm relative to the secondplate.

The MEMS pressure sensor of the preceding paragraph can optionallyinclude, additionally and/or alternatively, any one or more of thefollowing features, configurations, and/or additional components:

The first plate may be silicon.

The diaphragm may be silicon.

The isolation frame may be quartz.

The diaphragm and second plate may be electrically isolated.

The diaphragm may be bonded to the first plate by a fusion bond.

The isolation frame may be bonded to the diaphragm by a fusion bond.

The second plate may be bonded to the isolation frame by a fusion bond.

The cavity between the second plate, isolation frame, and diaphragm maybe under a vacuum.

A method of forming a MEMS pressure sensor may include forming a firstplate with a hole and forming a diaphragm by creating a depression in afirst surface of a second plate. The method may further include bondingthe first surface of the diaphragm to the first plate around a rim ofthe first plate and forming an isolation frame and bonding the isolationframe to the second surface of the diaphragm around a rim of theisolation frame. The method may further include forming a third platewith a pillar and a second surface of the third plate and bonding thesecond surface of the third plate to a third surface of the isolationframe such that an end of the pillar projects into a cavity that isformed and proximate the second surface of the diaphragm. The method mayfurther include forming a first metal electrode on an exterior surfaceof the diaphragm and forming a second metal electrode on a fourthsurface of the second plate to a form a capacitive sensor that changesoutput upon deflection of the diaphragm relative to the third plate dueto pressure differences communicated through the diaphragm through thehole in the first plate.

A method of the preceding paragraph can optionally include, additionallyand/or alternatively, anyone or more of the following features,configurations, and/or additional components:

The first plate may be silicon.

The diaphragm may be silicon and bonding the first surface of thediaphragm to the first plate may include fusion bonding.

The isolation frame may be quartz.

Bonding the isolation frame to the second surface of the silicondiaphragm may include fusion bonding.

The second plate may be silicon or SOI and forming the pillar in thesecond plate may be by dry reactive ion etching (DRIE).

Bonding the first surface of the second plate with the pillar to thesecond surface of the diaphragm may include fusion bonding.

The cavity between the second plate and the diaphragm may be evacuated.

A metal electrode formed on the exterior surface of the diaphragm may beAl, Ti, TiW, W, alloys or mixtures thereof.

The metal electrode formed on the second plate may be Al, Ti, TiW, W,alloys or mixtures thereof.

1. A MEMS pressure sensor comprising: a first plate with a central hole;a diaphragm bonded to the first plate around a rim of the first platewith the diaphragm positioned over the hole; an isolation frame bondedto a top of the diaphragm; and a second plate with a central pillarbonded to the isolation frame around a rim of the isolation frame toform a cavity such that an end of the pillar in the cavity is proximatethe first surface of the diaphragm; wherein the diaphragm and secondplate form a capacitive sensor that changes output upon deflection ofthe diaphragm relative to the second plate.
 2. The pressure sensor ofclaim 1, wherein the first plate is composed of silicon.
 3. The pressuresensor of claim 1, wherein the diaphragm is composed of silicon.
 4. Thepressure sensor of claim 1, wherein the isolation frame is composed ofquartz or glass.
 5. The pressure sensor of claim 1, wherein thediaphragm and second plate are electrically isolated.
 6. The pressuresensor of claim 1, wherein the diaphragm is bonded to the first plate bya fusion bond.
 7. The pressure sensor of claim 1, wherein the isolationframe is bonded to the diaphragm by a fusion bond, anodic bond, metaleutectic, or glass frit bond.
 8. The pressure sensor of claim 1, whereinthe second plate is bonded to the isolation frame by a fusion bond. 9.The pressure sensor of claim 1, wherein the cavity between the secondplate, isolation frame, and diaphragm is under a vacuum.
 10. A method offorming a MEMS pressure sensor comprising: forming a first plate with ahole; forming a diaphragm by creating a depression in a first surface ofa second plate; bonding the first surface of the diaphragm to the firstplate around a rim of the first plate; forming an isolation frame;bonding the isolation frame to a second surface of the diaphragm arounda rim of the isolation frame; forming a third plate with a pillar in asecond surface of the third plate; bonding the second surface of thethird plate to a third surface of the isolation frame such that an endof the pillar projects into a cavity that is formed and is proximate thesecond surface of the diaphragm; forming a first metal electrode on anexterior surface of the diaphragm; and forming a second metal electrodeon a fourth surface of the second plate to form a capacitive sensor thatchanges output upon deflection of the diaphragm relative to the thirdplate due to pressure differences communicated to the diaphragm throughthe hole in the first plate.
 11. The method of claim 10, wherein thefirst plate is composed of silicon.
 12. The method of claim 10, whereinthe diaphragm is composed of silicon, and bonding the first surface ofthe diaphragm to the first plate comprises fusion bonding.
 13. Themethod of claim 10, wherein the isolation frame is composed of quartz orglass.
 14. The method of claim 10, wherein bonding the isolation frameto the second surface of the silicon diaphragm comprises fusion bonding,anodic bonding, metal eutectic bonding, or glass frit bonding.
 15. Themethod of claim 10, wherein the second plate is silicon or SOI, andforming the pillar in the second plate is performed by dry reactive ionetching (DRIE).
 16. The method of claim 10, wherein bonding the firstsurface of the second plate with the pillar to the second surface of thediaphragm comprises fusion bonding.
 17. The method of claim 16, furthercomprising evacuating the cavity between the second plate and thediaphragm.
 18. The method of claim 10, wherein the metal electrodeformed on the exterior surface of the diaphragm comprises Al, Ti, TiW, Walloys or mixtures thereof.
 19. The method of claim 10, wherein themetal electrode formed on the second plate comprises Al, Ti, TiW, Walloys or mixtures thereof.